As known, a much-used wrapping technique provides to wrap tightly a load to be transported with one or more strap loops. The strap is a thin tape, normally of plastic material (but in some cases also of metal), which is tightly wound loop-like around a load, closing it permanently by means of welding points between the two terminal lips.
The welding avoids a reopening of the strap loop, which must hence then be severed to free the wrapping.
Welding types can vary, also depending on the type of material of the strap. However, all strapping machines substantially resort to a strapping head which has two complementary and integrated functions: on the one hand, the launch and subsequent rewind of the strap with tightening around the product to be wrapped and, on the other hand, the welding of the two terminal lips.
Normally a single strapping head, mounted below the load-transit plane, embeds a series of motion devices and mechanism which perform all the above-said functions, that is, they feed and launch the strap around the load, starting from a storage spool, they block the free end and retrieve the base portion, until choking a strap loop around the load; finally, they perform the welding in the loop closing area and severe it from the rest of the strap coming from the spool.
This assembly is rather complex and bulky. As a matter of fact, it comprises various transmission members, actuation cams, rotation mechanisms and motors, all the above mounted on a sturdy containment frame. That causes the entire head to weigh several tens of kilograms and is hence cumbersome to move. Vice versa, for various maintenance operations, it would be desirable to be able to easily disassemble and displace the head, so as to replace it easily or to perform the necessary inspections with ease.
For such purpose, it has already been suggested to divide the strapping head into at least two main assemblies, one delegated to the strap launching and retrieving function, with the relative motorisation, the other delegated to strap welding and cutting, with relative motorisation. The disassembling into two assemblies has the advantage of dramatically improving the opportunity for inspection and for replacement. Some examples consistent with this approach are represented by EP1275586 and by the Italian application MI2010A2231 in the name of the same Applicant.
Although some of these systems are advantageous, it has been realised that the user of these apparatuses would in any case wish to be able to have—at least in the least burdensome applications—a single strapping head, preferably compact and light so as to be able to be held comfortably.
The Applicant has hence further focused on the problem of making an integral strapping head light and compact. When faced with such problem, it has become apparent that a construction constraint of known heads is that of having to displace with rotating shafts (those of electric motors) both elements with a longitudinal main movement (the strap launching and rewinding axis) and elements with a crosswise main movement (the components of the welding system). That implies providing cam transmissions on orthogonal planes, which have a significant space occupation, determine project constraints and, due to the cyclicity thereof on the single motor shaft revolution, require complex transmission rotation mechanisms to define the motion laws suited to the various displacement members. All that ends up having a negative influence both on manufacturing costs and on apparatus weight.
The prior art offers rare examples of strapping heads wherein use is made of controls of an alternative shape. One of these is represented by U.S. Pat. No. 3,759,169, wherein part of an actuation is entrusted to a linear cam, instead of to a classic rotating cam; in particular, the control of the strap retaining grippers is entrusted to a pneumatic actuation through a single linear cam where displacement of the follower is made only on a vertical plane (i.e. orthogonal with respect to the plane of displacement of the grippers). However, these known solutions have not proved effective because they comprise mixed actuations and hence overall they have significant bulks, as well as having a pneumatic control, hence they are capable of expressing a low force, are little controllable and little accurate in position.